The black hole binaries detected so far by the LIGO-Virgo Collaboration were “clean” sources of gravitational waves : the binary dynamics and signal were solely determined by gravity itself. Next generation gravitational-wave detectors, such as LISA, will in addition be sensitive to a number of “dirty” sources. For instance, astrophysical processes like mass transfer could dominate the dynamics of some galactic binaries, such as white dwarf-black hole binaries. Even the signal from black hole binaries, if the latter are located in active galactic nuclei, could be affected by a long list of environmental effects, from dynamical friction to accretion, to acceleration.
In this talk, I will give a brief overview of environmental effects in gravitational waves. I will then describe in more detail the two examples mentioned above, and discuss what we can hope to learn from these messy signals.